I recently made up a usb thumb drive with Lupu 525, Lupu 528, and Precise 550, using grub4dos to select the OS. All boot fine on my Thinkpad T43, but Precise (only) wouldn't boot on my Asus M4A79T Deluxe. The symptom is exactly as described in this thread.
The workaround mentioned here - deleting the "pmedia=usbflash" from the menu.lst file - does make it boot. I can hear the disks thrashing for several seconds before Precise finds itself. However, I would think that there should be a better and more portable way to kill some time for the usb drive to wake up.